As mobile stations are multi-functionalized, ways of using the mobile stations by users are diversified. The mobile stations are represented by mobile phones. Whereas, operators are providing benefits to the users by offering various charge plans to the users.
There has been introduced a system that simulates monthly charge plans for mobile phone subscribers as a web service or at a store. When a subscriber is to purchase a mobile phone or to decide or change a charge plan, the subscriber may find a charge plan that is suitable for the subscriber in advance by performing the simulation. For purchasing a mobile phone and for deciding on a charge plan, one of important matters of concern for the subscriber is how long a battery of the mobile station can be used, namely, “the battery usable time period.” Patent Document 1, for example, discloses a conventional technique for calculating a remaining amount of the battery usable time period.